1. Field of the Invention
The present invention relates to the determination of behavioral models for nonlinear devices, circuits, subsystems or systems. In particular the present invention is a method for creating excitation signals and a method for performing function fitting in the determination of behavioral models for nonlinear devices, circuits, subsystems, and systems from embeddings of time-domain measurements.
2. Description of the Related Art
Linear, time invariant (LTI) devices, circuits, subsystems, and systems are completely characterized by their transfer functions. To understand the performance of an LTI device, one need only determine the transfer function of the LTI device. Once the transfer function is known, the operation of the device in a system is known completely for all input conditions. The same is true for LTI circuits, subsystems and systems.
A transfer function is a complex frequency domain-function that describes the output of an LTI device in terms of its inputs and therefore, forms a complete description of the LTI device. The term complex function when used herein refers to a function that includes complex numbers having a real and an imaginary part. An equivalent form of the transfer function of an LTI device in the time-domain is called an impulse response of the LTI device. A one-to-one relationship exists between the transfer function in the frequency-domain and impulse response in the time-domain. In addition, the transfer function and the impulse response are not functions of and do not depend on the input signal that is applied to the LTI device.
The determination of the transfer function, especially if it involves measured data from the LTI device, is known as model development or model parameter extraction. Once a model of an LTI device is developed, or equivalently the transfer function is known, for a given device, the actual device may be replaced by a virtual device based on the model in any simulation of a system using the device. Often the development of the model involves extraction or determination of model parameters from a set of test data that represents the device of interest.
Transfer functions of LTI devices, circuits, subsystems, or systems can be extracted from measurements made with a vector spectrum or vector network analyzer. A swept or stepped frequency input signal is generated and the vector spectrum analyzer or network analyzer records the output of the LTI device. Then, a transfer function can be computed by comparing the input and output signals. Furthermore, models suitable for simulation of a given LTI device or circuit can extracted from transfer functions using, among other things, linear system identification techniques.
Time-domain measurements provide an alternate method of characterizing LTI devices or circuits. Pulse inputs that approximate an impulse are applied to a device and the outputs are measured and recorded. In one such well known, time-domain method, the poles and zeros of the Laplace transform of the governing differential equation of the device are estimated from the recorded output data. Once a suitable governing differential equation is determined, the device transfer function is calculated. In an alternative method, the measured data associated with the impulse response is transformed using a Fast Fourier Transform (FFT) to the frequency-domain where a linear system identification method is then used to extract the transfer function.
The characterization or modeling of nonlinear devices or circuits is much more difficult than that for LTI devices. Reference to a xe2x80x9cnonlinear devicexe2x80x9d when used herein will be understood to include devices, circuits, subsystems or systems with a nonlinear input-output relationship. Unlike the linear case, the nonlinear device or circuit is not readily represented by a transfer function or impulse response, at least not one that is independent of the input signal or stimulus. However, there is still a need to model nonlinear devices so that their performance in systems can be evaluated efficiently. This is especially true when it is impractical or too expensive to use the actual device, such as when the device is still being designed.
It is desirable to have a method for characterizing and developing a model of nonlinear devices to avoid the need to have the actual device available whenever its performance in a system must be investigated. Furthermore it is advantageous to have such a modeling method utilize a finite set of measurements, either actual measurements or measurements of a simulation of the device. The model so generated must accurately predict the performance of the device over all expected operational conditions within a given level of accuracy and with an acceptable amount of computational cost.
The term xe2x80x9cbehavioral modelxe2x80x9d herein refers to a set of parameters that define the input-output behavior of a device or circuit. Generally, a behavioral model must be of a form suitable for rapid simulation. xe2x80x9cSimulated measurementsxe2x80x9d refers to values of voltage, current or other physical variables obtained from device, circuit or system simulation software. The objective of building a behavioral model from actual or simulated measurements is to reduce simulation time by replacing a complex circuit description in the simulation with a simpler, easier to simulate, behavioral model.
In many cases, nonlinear devices are electronic in nature (e.g. transistors, diodes). In these cases the measurements used to produce a model of the device are typically measured voltages and currents in and out of the ports of the device or equivalently incident or reflected power waves present at the ports at various frequencies. The models extracted from the measurements generally need to reflect the dynamic relationships between the voltages and currents at the ports. The model can be used, for example, to compute the currents into the ports from recent values of the voltages across the ports. Often this is the essential computation that must be provided to electronic circuit simulators by a software module that represents a device.
Mechanical and hydraulic devices can also exhibit nonlinear behavior and, therefore, be modeled as nonlinear devices for which construction of a suitable behavioral model would be beneficial. For example, a vehicular system comprising driver inputs and vehicle response may be represented in terms of a nonlinear behavioral model. In the case of vehicular systems, the input measurements might be of variables such as steering wheel position, brake pressure, throttle position, gear selection and the response measurements might be of variables such as the vehicle speed, lateral and longitudinal acceleration, and yaw rate. The behavioral model extracted from the measurements needs to reflect the dynamic relationship between the driver inputs that are applied and the subsequent response of the vehicle. In other words, the model defines a xe2x80x9cvirtual carxe2x80x9d that can be xe2x80x9cdrivenxe2x80x9d using previously recorded or real-time measured driver inputs.
A variety of methods have been developed to characterize and develop models of nonlinear devices. However, these methods generally have severe limitations associated with them. For instance, many of the techniques are limited to use with so called xe2x80x9cweakly nonlinear devicesxe2x80x9d, those devices whose performance is nearly linear. Therefore, these techniques are not suitable for many nonlinear devices.
One such approach to characterization of weakly nonlinear devices is to simply assume that the device is linear, at least in the operational range of interest. Under this assumption, a variant of the time-domain impulse response method described hereinabove can be used. For devices that are, in fact, weakly nonlinear devices, this approach yields reasonably good results. However, the accuracy of such a model will degrade rapidly as the amount or degree of nonlinearity in the device increases.
Another class of methods for characterizing nonlinear devices is represented by the Volterra input-output maps method (VIOMAPs) also known as the Volterra Series Method. VIOMAPs are models of nonlinear devices or circuits that can be extracted from frequency domain measurements such as those produced by using a vector spectrum analyzer. Here again, the usefulness of such models is limited by the assumption of weak nonlinearity. In addition, VIOMAPs and related methods can only model the steady state behavioral or response of the device. A steady-state response is the response of a device, linear or nonlinear, to a repeating input signal. An example of a steady-state response is the response to a sine wave input after sufficient time has passed to allow the transients associated with the application of the sine wave to decay. VIOMAPs and the related methods are powerful methods that have found many useful applications. However, VIOMAPs, as noted above, cannot handle strong nonlinearities or transient inputs. VIOMAPs are restricted to modeling the steady state behavior of devices that exhibit weak nonlinearities.
Another, somewhat different, method of characterizing nonlinear devices is to use an equivalent circuit representation of the device of interest. The approach in this method is to assume an equivalent circuit topology with a certain circuit parameter left free or unspecified that is expected to adequately represent the device or circuit. For example, equivalent circuits are known that adequately represent certain classes of transistors (e.g. MOSFETs or BJTs). Given the assumed equivalent circuit, a set of measurements is performed on the device from which the correct values of the free parameters can be computed or deduced for a particular device.
As with the other methods, this approach for nonlinear device characterization has a number of serious disadvantages. Chief among the disadvantages is the need for a priori knowledge of an equivalent circuit that adequately represents the device of interest. This often means that significant knowledge of the device is required before modeling can begin. If incorrect assumptions are made regarding the structure of the equivalent circuit, the method may not yield satisfactory results. Put another way, the approximation that is being made by choosing a particular equivalent circuit over another has an impact on accuracy that is hard to determine. In addition, this method is only useful when the device being modeled is of a type similar to electronic circuitry (e.g. a hydraulic device or spring-mass-dashpot system) that a representative equivalent circuit can be created. Finally, the equivalent circuit can require a significant amount of computer time to simulate, thereby often making this method unacceptably costly for use in the simulation of large systems.
Therefore, it would be desirable to have a method for the construction of a behavioral model of a nonlinear device that is not limited to assuming the device is weakly nonlinear and that does not require excessively large amounts of computational effort to produce simulated results. In addition, it would be desirable if this method were not limited to steady-state response characterizations and this it method did not require any a priori knowledge of the device being modeled. Moreover, it would be advantageous if this method would allow the model to be constructed from either actual measurements or simulated measurements of the device. Finally, it would be advantageous if this method utilized an excitation signal and employed a functional fitting technique that were broadly applicable to many nonlinear devices. Such a nonlinear characterization and model construction method would overcome a long-standing problem in the area of nonlinear device modeling technology.
The present invention is based on the method for producing behavioral models of nonlinear devices that is described in co-pending application Ser. No. 09/420,607, filed Oct. 18, 1999. In particular, the methods of the present invention are a method for producing behavioral models of nonlinear devices utilizing a robust excitation signal, a method for producing behavioral models of nonlinear devices utilizing a novel radial basis function, a method for constructing the robust excitation signal, and a method of determining a novel radial basis function. The behavioral models resulting from the methods of the present invention accommodate nonlinear devices with one or more input ports and one or more output ports.
The methods of producing a behavioral model of a nonlinear device from embeddings of time-domain measurements of the present invention comprise the steps of applying an input signal to the nonlinear device, or equivalently to a virtual device, sampling the input signal to produce input data, measuring a response to the input signal at the output of the device to produce output data corresponding to the input data, creating an embedded data set using a first subset of the input data and a first subset of the output data, fitting a function to the embedded data set, and verifying the fitted function using a second subset of the input data and a second subset of the output data, wherein the verified fitted function is the behavioral model of the nonlinear device. In another embodiment, the fitted function can be used to compute a continuous-time model from the discrete behavioral model developed in the aforementioned steps.
In one embodiment, the method producing a behavioral model of the present invention utilizes a code division multiple access signal (CDMA) as the input signal in the step of applying. In another embodiment, the method of producing a behavioral model of the present invention utilizes a novel radial basis function in the step of fitting a function to the embedded data set. In yet another embodiment, the method of producing a behavioral model utilizes both the CDMA type signal as an input signal in the step of applying and the novel radial basis function in the step of fitting.
Unlike the aforementioned conventional methods, the methods of the present invention are not restricted to modeling weakly nonlinear devices but can handle hard or strong nonlinearities. The methods accommodate steady-state as well as dynamic nonlinearities, requires no a priori assumptions regarding the device model structure and can handle devices with multiple, dependent inputs. Further, the methods operate in the discrete time-domain with sampled measurements of input and output variables but are readily extended to a continuous representation of the device behavioral model. As such, the behavioral models produced by these methods are general in nature and are readily implementable representations of nonlinear devices including those exhibiting strong nonlinearities.
Moreover, the behavioral model created from the methods of the present invention can be used to simulate the output of the device, given input signals in the frequency-domain. The excitation signals utilized can be generated at reasonable cost at any frequency up to and beyond microwave frequencies.
In another aspect of the present invention, a method of constructing an input or excitation signal for use in producing a behavioral model is also provided. The method of construction of the invention constructs a CDMA type excitation signal. The excitation signal so constructed is applied to a nonlinear device for behavioral model extraction from embedded time-series measurements according to the invention.
The CDMA type signal is a single signal as opposed to multiple signals typical of the excitation signal constructed according to the co-pending method. Advantageously, the single CDMA type signal of the present invention provides excellent phase space coverage. Therefore, use of the CDMA type signal in the methods of the present invention yields models with excellent agreement between the behavioral model produced thereby and the actual nonlinear device.
In yet another aspect of the present invention, a method of determining a radial basis function for use in determining a behavioral model is provided. The radial basis function comprises a novel modified gaussian function. When used in the production of a behavioral model in the methods of the present invention, the novel radial basis function of the present invention provides excellent agreement between the behavioral model and the nonlinear device being modeled. In addition, the radial basis function is stable owing to the inherent partitioning of the modified gaussian function into two parts, one part associated with output data from the model and the other part associated with input data from the excitation signal.
In yet another aspect of the present invention, the CDMA type excitation signal and the radial basis function of the present invention can be used together in the method of producing a behavioral model of the present invention.